Low emissions catalytic combustion system

ABSTRACT

In a continuous combustion, regenerative, gas turbine engine, supply of compressed air is divided so that a portion thereof can be mixed with fuel to support a local flame on the surface of a heat exchanger through which all portions of the flow pass to be combusted. The flame brings the temperature of the flow up to a predetermined operating range for supporting catalytic combustion and at which time the local flame is extinguished.

a v United States Patent [1 1 [111 3,797,231 McLean Mar. 19, 1974 [5 LOW EMISSIONS CATALYTIC 2,667,034 1/19s4 Alcock 60/3951 H COMBUSTION SYSTEM 3.641.763 2/1972 Cole 60/3951 H [75] Inventor: Arthur F. McLean, Ann Arbor, FOREIGN PATENTS OR APPLICATIONS Mich. 741.506 12/1955 Great Britain 60/3982 C d M c D b [73] Asslgnee i 32 otor ampany ear Om Primary Examiner-Carlton R. Croyle Assistant Examiner-Warren Olsen [22] Filed: July 31, 19 2 Attorney, Agent, or Firm-Keith L. Zerschling; Joseph 21 App1.'No.: 276,875 Mane i 57 ABSTRACT [52] US. Cl 60/3951 H, 60/3982 C, 60/300, 1

431/328 In a continuous combustlon, regenerative, gas turbine [51] Int. Cl. F02c 7/10 engine Supply of Compressed air is divided so that a 58 Field of Search 60/3951 H, 39.82 c, 300; "i there can be mixed with fuel a 1 431/328 local flame on the surface of a heat exchanger through g which all portions of the flow pass to be combusted. [56] Referefices Cited The flame brings the temperature of the flow up to a UNITED STATES PATENTS predetermineel operating range for supporting catalytic combustlon and at WhlCh time the local flame 1s 3.191.659 6/1965 Weiss 431/328 extinguished I 3.563.031 2/1971 Topouzian 60/3951 H 3.182.472 9 Claims, 3 Drawing Figures 5/1965 Toone et a1 60/3982 C t [I //m w W 992 i Eff mullfi mm! i 4/ f --1/ /j 5;

Pmmwmm 1914 3797.231

' SHEET 2 0F 2 LOW EMISSIONS CATALYTIC COMBUSTION SYSTEM BACKGROUND OF THE INVENTION Catalysis, as a means of supporting combustion without a flame and without entering chemically into the combustion reaction, is well known and is the'foundation of many useful industrial applications. But there has been little or no practical applications of catalysis as the primary means for adding working heat in continuous combustion engines, particularly in gas turbine engines. Appropriate utilization of a catalytic combustion mechanism, as taught by this invention, provides immediate advantages including: reduction of unwanted emissions such as nitrogen oxide compounds to levels below two parts per million; provision of a more even temperature gradient across the hot gases generated by catalysis for driving a turbine wheel and thereby increasing engine life.

Certain problems arise when employing catalysis as a primary heat producer. First, there is an inability to start from relatively cold ambient temperature conditions for the operating air/fuel ratios contemplated. The typical air/fuel mixtures utilized to support combustion in flame-type combustion assemblies, and after operating temperature is reached, range appropriately from 80:] through 190:1 whereas fuel ratios for supporting catalytic combustion do not have a technical maximum limit. For example, effective catalytic combustion can take place with an air/fuel mixture in the range of 100021, but practical air volume supply 'requirements may necessitate ratios of 150:1 to be used. All of these latter ratios are too lean to be ignited and support a flame for prewarming purposes. As a practical target at room temperature conditions, the air/fuel ratio must be between 8:1 to 30:1 at the point of combustion to hold a flame upon ignition and thereby provide some initial prewarming mechanism to bring the heat exchanger and air/fuel mixture up to normal operating conditions to sustain catalytic combustion.

SUMMARY OF THE INVENTION This invention provides an apparatus for adding primary heat to a continuous combustion process using a catalystic combustion mechanism effective to promote sustained-combustion with lean or exceptionally lean air/fuel mixtures, the mixture being prewarmed by a regenerative means transferring heat from combusted gases to the unburned mixture. For purposes of starting such an engine from cold conditions, preheating means is employed to divide the air supply and introduce fuel to this divided portion for regulating andpromoting a predetermined air/fuel mixture effective to support a flame when ignited. This divided mixture in one embodiment can be passed through a predetermined radial region of the regenerator operating as a rotating disc, the mixture being ignited to support flaming combustion adjacent to the regenerator. The flame is anchored on the regenerator by a surrounding cylinder of air and the flame is effective to bring the system up to a normal operating temperature. The divided mixture in another embodimenucan be ignited considerably downstream of the regenerator heat extraction zone in an auxiliary combustor assembly effective to flamingly burn substantially all or majorly the air/fuel mixture air/fuel ratio (at or below 30:1) for supporting a preheating flame at ambient air temperatures and consuming the mixture flowing from the duct. The duct mixture is directed through a regenerator matrix which insures totality of fuel vaporization; to prevent carryover of any unvaporized fuel beyond the heat extraction zone of the regenerator, the duct is located off center from a D-shaped passage communicating with one half of the rotating regenerator for defining the heat extraction zone. The offset is in a counterclockwise direction relative to the direction of rotation of the regenerator.

If the embodiment has divided the primary combustion air supply, remixing of the divided portions can be promoted in a plenum zone upstream of the catalytic combustor but downstream of the-regenerator. This is facilitated by the use of swirl vanes or vortex generators introducing a secondary air supply to the plenum zone.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of a portion of one engine embodying this invention employing a midradial region duct 'for dividing the primary air flow;

FIG. 2 is a graphical illustration comparing air stream temperature-to air/fuel ratio; and

FIG. 3 .is a schematic illustration of another embodiment employing an auxiliary combustion chamber as a flow divider.

DETAILED DESCRIPTION Referring to FIG. 1, a continuous combustion system is illustrated with reference to a preferred gas turbine engine comprising a housing assembly 10 for defining a flow system 8 through the engine; the housing has an air inlet 11 at one end generally aligned with the axis 12 of the flow system. A centrifugal compressor 13 is mounted inside air inlet 11 for generating a supply of compressed air; the compressor is connected by a shaft 14 to a compressor turbine wheel 15, the wheel being suitably supported in housing 10 for rotation. A power turbine wheel 16, constituting the working element of the system, is rotatably mounted behind compressor turbine wheel 15 and is independently connected to an output shaft 17. The flow system is arranged to act upon both turbine wheels immediately following the combustion zone 9.

Means for adding heat and fuel to the stream of air flowing from said inlet 11 broadlycomprisemeans for providing catalytic combustion A, preheating means 13, regenerator C, and fuel supply means D. The housing and cooperating internal walls 19 define a flow path through the system into which the means A, B, C and D project. In more particularity, housing 10 extends radially outwardly from the tips of the blades of compressor l3 (and a diffuser element not shown) and extends curvingly backward over the forward half-circular portion 20a of disc 20. Disc 20 constitutes a disc-type regenerator (means C) preferably made of ceramic material for providing a matrix or plurality of small passages running therethrough in a direction generally aligned parallel to the rotational axis 21 of the disc. The housing, after having curved backward over the forward semi-circular portion 20a then curves inward along a diameter of disc 20 perpendicular to axis 21 to form path portion 22. When viewed in elevation, the flow portion 22 is D-shaped so as to enter one-half the circular area of disc 20.-

A system path portion 23 is defined by ceramic internal walls 19 disposed downstream of the regenerators 20 having an opening 24 therein for operatively and nestably receiving said rotatable turbine wheels and 16; path portion 23 traverses the rear semi-circular portion b of discs 20.

The catalytic combustion means A comprises a honeycomb ceramic element 25 which can be conically or cylindrically shaped to occupy a major region of the plenum area 26. The ceramic honeycomb is coated with a suitable catalyst for promoting combustion of an air/fuel mixture at temperatures contemplated herein. The honeycomb has a configuration effective to cover the total throat area 53 for providing an effective and uniform temperature gradient thereacross.

The preheating means B comprises in part a duct of curved tubular construction having a circular inlet 31 in a central location of path portion 22. An outlet 32 of the duct is juxtaposed surface 33 of the ceramic regenerator disc 20. The duct is effective to divide a portion of the primary air, approximately in a proportion of Va to and directs this separated or divided portion to flow through a mid-radial region 34 of the regenerator. Duct outlet 32 may be offset in a direction upwardly from the plane of the paper of FIG. 1; the offset would thus be in a counterclockwise direction of the disc 20 if rotating as arrows indicate in the figure. This offset decreases the possibility that fuel will be rotationally carried by the disc into the flow downstream of the turbine wheels without passing'through the combustion zone. An ignitor 38 extends into the plenum area 26 and is arranged to ignite the air/fuel mixture flowing out of the radial region 34 of the regenerator. The resulting flame will be localized and is anchored at a slight distance (about 2-3 mils) off of surface 39 of the regenerator; anchoring of the flame is assured by the cylinder of air (containing no fuel) passing through other regions of the regenerator. Heat produced by the localized flame 40 is absorbed in part by the forward portion 20a of the regenerator and absorbed in part by the flow which conducts heat to the downstream portion of the regenerator. After the preheating means has been operating for a period of time, preferably 4-9 seconds, the regenerator and air temperature will rise to levels at or above 400F which condition is capable of sustaining catalytic combustion in means A.

Regenerator means C should be of the rotating type because normal inlet temperatures to sustain catalytic combustion may become excessive for'stationary types. Depending on the nature of the ceramic matrix of the regenerator, it usually must be kept at a temperature below 1950F to avoid minute cracking resulting from excessive expansion; this is facilitated by rotation which assures a more rapid heat exchange preventing excessive temperature buildup. It also is important for efficiency that all fuel be vaporized in the interstices of the regenerator and be homogeneously mixed with air. The offset location of duct 30 relative to the regenerator facilitates the latter. Air not divided and not passing through duct 30 will pass freely through the outer regions of the regenerator forming a cylinder of air in plenum area 26 about the mixture emitted from duct 30. Under conditions of sustained catalytic combustion, the cylinder of air is mixed with the duct mixture and together enter the catalytic combustion means A. This may be accomplished by swirl vanes or vortex guides 50 receiving secondary air from a channel source 51 so as to introduce the secondary air to turbulently mix the remainder of primary air with the fuel mixture.

The fuel supply means D which functions for the total support of both initial combustion and sustained catalytic combustion has an outlet 41 extending through the housing 10 into duct 30 whereby fuel droplets may be dispersed effectively inside the duct possessing air at a relatively low temperature even at normal operating conditions (150F). This low temperature location promotes the use of low cost fuel nozzles such as the pintle type which is characterized by excellent atomization. Control valve 42 is provided to interrupt full fuel flow to extinguish the localized flame 40 when catalytic operating temperatures are reached.

The entrance face 28 to the catalytic combustion means is preferably adapted to operate with the incoming fuel mixture preheated to the temperature range between 1500F to 1700F. Fuel mixtures at lower temperatures passing into the matrix of the catalytic combustor will sustain continuous combustion but engine efficiency is affected. The air/fuel ratio of the mixture passing through face 28 can be in an exceedingly high range such as l000:1 although practical limitations of air supply appropriately place the ratio at to 250:1. The outlet temperature of emissions from the catalytic combustor is preferably about 2000F and any excess temperature should be controlled to be less than 2800F which is the maximum limit over which the catalytic matrix will be stressed; moreover at 3000-3200F, undesirable nitrogen oxide compounds will form in sizable amounts. Combustion gases having a temperature above 2000 may unduly stress the material of which the turbine wheels are constituted requiring an air quench immediately between the entrance to the turbine wheels and the catalytic combustor.

In operation, the engine of FIG. 1 is started by introducing ambient air to the inlet 11 of housing 10 wherein it is compressed by centrifugal compressor 13 and caused to flow along path portion 22 where it becomes divided by duct 30 into predetermined proportions, the portion passing through duct 30 having a supply of fuel added to it by fuel supply means D. The divided portion within duct 30 is conveyed to a midradial region of the regenerator to exit also at a substantially mid-radial region from the surface of the regenerator. At the same time, other portions of the air pass through the outer and inner radial regions of the regenerator forming somewhat a cylinder about the mixture centrally located therein. Fuel added to the duct is measured to provide an air/fuel ratio that will meet the conditions as illustrated in FIG. 2. Combustion can take place with or without an ignition source (spark) depending on the temperature and chemical constituency of the mixture. However, under certain combinations of air/fuel ratios and temperatures, no flaming combustion will be sustained even with an ignitor (areas outside the horizontal hatched area of FIG. 2 enclosed by line 45 and lean blow-out line 46). Test data indicates that at typical ambient temperatures conditions (50F) the limits of air/fuel must lie within 8/ l to 30/1 to support a continuous flame. A high degree of reliability in ignition can be obtained by utilizing a mixture in the area bounded by line 47 (/1 to 25/1). Accordingly, the duct 30 should be related to the fuel supplied to provide a mixture ranging between 15:1 to 25:1 to be absolutely assured of preheating when starting from cold conditions. However, for better efficiency of preheating, the ratio should be maintained close to the lean blow-out line 46, hereby being closer to 30:1. Once air temperature in the system reaches or exceeds the minimum catalysis temperature, line 48, sustained catalysis can be provided regardless of air/fuel ratio.

lgnitor means D is employed to spark a flame as the mixture exits from the regenerator to combust the mixture. The flame at certain points may have a temperature as high as 3500F which provides a quick source of preheat. At this starting condition of the engine, the

catalytic combustor means A is not functioning other than-to pass air and fuel/air mixtures therethrough. It is also not important at this stage whether the cylinder of air is homogeneously mixed with the combusted mixture in that the immediate aim is to raise the operating temperature of the regenerator as well as the air flow ultimately contacting other portions of the regenerator; this can be effectively accomplished by conduction irrespective of mixing. Typically it takes a period of 6-9 seconds for the temperature rise to exceed the level of 400F (which is the minimum temperature condition for sustaining typical catalysis see FIG. 2). After suitable signals have indicated that the target temperature (of the air stream passing through the catalytic combustor) has been reached, controls (not shown) are employed to extinguish the flame by temporarily cutting off the fuel supply. Such controls can at the same time determine the combustion activity of the catalytic means A before such flame is extinguished.

After sustained catalytic combustion is attained, it is important that the total air supply be turbulently mixed downstream of the regenerator so that the available fuel (derived from the fuel supply means D injecting into the duct 30) can obtain a relatively homogeneous and uniform mixture with other air portions prior to entrance into the catalytic combustion means. As indicated, swirl vanes or vortex generators disposed in the walls of the plenum area can introduce a secondary air supply at this stage of the operation.

Conventional means may be utilized to increase or decrease the fuel supply rate as well as the speed of the compressor 13 to maintain a uniform catalytic combustion temperature in conformity with a desired speed of the working turbine 16.

An alternative embodiment is illustrated in FIG. 3 which essentially provides an auxiliary combustion chamber 49 to support flaming combustion in advance of the catalytic combustion means and acts as the flow divider per se. More particularly this gas turbine engine provides for an intake of air through an inlet duct 53 directed in a radially inward direction of the engine and is disposed rearwardly of the primary combustion chamber 54. Interior wall system 55 defines a flow path 56 which enters a centrifugal compressor 57 disposed at a central inner region about axis 58. Flow from the compressor returns radially outwardly through a diffuser 59 and the air stream communicates to both sides of the engine at which regenerative rotating discs are located. Inner walls 55 conduct the compressed air to the forward half of the regenerative disc permitting it to pass therethrough similar in function to that of the preferred embodiment. Interior wall 61 and the outer surface of the cylinder 62 carry the compressed air to the most forward portion of the engine to enter the conical dome 63 of the auxiliary combustion chamber 49 (this would normally be a primary combustion chamber in typical prior art). The combustion chamber 49 may have a diameter substantially equal to the outer diameters of twin turbines 64 and 65 (with adjacent stators 77 and 78) disposed at the exit thereof. A pilot auxiliary chamber 66 (shown in phantom outline) may alternatively be used inside of dome 63 to restrict flame by virtue of a lesser diameter and is disposed inwardly of the outer chamber 49. Fuel supply means 67 is located so as to inject a conventional supply of fuel into the area of dome 63 or into the pilot auxiliary chamber 66 if used; an ignition means 68 is employed to spark in the region of the dome 63 or inside chamber 66 if used.

The primary air, needed to support the preheating flaming combustion, enters the auxiliary chamber 49 through ports 69 adjacent the fuel supply means, and a divided air flow portion enters somewhat downstream at ports 70 to mix and generate turbulence within the auxiliary combustion chamber. Once the temperature of the air system has increased to 400F or greater, the fuel supply means is interrupted so as to extinguish the flame 71 in the auxiliary combustion chamber 49 or pilot chamber 66, and then reopened to provide a leaner air/fuel mixture than that used during preheating combustion. Secondary air, as well as the divided pri mary air, is then conveyed to the primary combustion chamber 54 where a catalytic combustor means 73 is located forward of the entrance 75 to the twin turbine wheels. To control exit temperature of the emissions from the catalytic combustor, a still further divided portion of the primary air may be admitted through ports 74 to create air dilution and quenching.

Yet another modification of the preferred embodiment may be obtained by utilizing a vane plate 76 (see FIG. 1, shown in phantom outline) in place of the duct, the flow is divided proportionally to one forward side of the plate with fuel supplied only to this divided por- 1 tion.

I claim as my invention:

1. For use in a continuous combustion system having means for producing a primary stream of compressed relatively cool air to which heat and fuel is added to define a mixture for supporting combustion and thereby generating hot gases for driving a working element, apparatus for adding heat and fuel to the stream of compressed air comprising:

a. means for adding fuel 'to said stream of air in a manner providing a homogeneous air/fuel mixture at a predetermined temperature for generating said hot gases,

b. catalytic combustion means effective to sustain continuous, flameless oxidation of said mixture at a predetermined temperature, thereby generating said hot gases,

c. heat exchange means for transferring heat from said hot gases to the stream of compressed air, said heat exchange means being comprised of a perforate rotating member having surfaces effective to contact separately both said hot gases and air stream during a single rotation for transferring heat therebetween,

d. preheating means located upstream of said catalytic combustion means for raising the temperature of said mixture to said predetermined temperature in advance of sustained catalytic combustion by regulating said air/fuel mixture for supporting independent flaming combustion of said mixture upstream of said catalytic combustion means, said preheating means comprising a duct effective to separate a portion of said air stream and direct said portion through one radial region of said member, said preheating means being adapted to add fuel solely to said portion, said preheating means also having ignition means for igniting said regulated mixture and having control means for extinguishing said flaming combustion when said mixture attains said predetermined temperature for sustaining catalytic combustion, the ignition means being located proximate to at least one surface of said perforate member for igniting said mixture immediately downstream of said member.

2. The apparatus as in claim 1, in which said perforate member is particularly defined as a disc having a ceramic matrix with flow passages therethrough disposed generally transverse to the axis of rotation of said disc, and said preheating means being arranged to anchor said flame to one side of said disc immediately adjacent the passages thereof which carry said mixture from said duct.

3. The apparatus as in claim 2, which further comprises means for providing a cylinder of air to anchor said flame.

4. The apparatus as in claim 1, in which the air/fuel mixture passing from said duct into said heat exchange member is between 8:1 and 30:1.

5. An apparatus as in claim 1, in which said air stream is directed to pass through the regenerator on one side of the rotational axis thereof, said duct having an outlet adjacent said regenerator and offset counterclockwise relative to the rotation of said regenerator and from a line dividing said one generator side.

6. An apparatus as in claim 1, in which the unseparated portions of said air stream is effective to pass through said regenerator and define a cylinder of air about the air/fuel mixture exiting from said duct, said air cylinder being effective to anchor the flame resulting from the combustion of said mixture.

7. For use in a continuous combustion system having means for producing a primary stream of compressed relatively cool air to which heat and fuel is added for supporting combustion and thereby generating hot gases for driving a working element, apparatus for adding heat and fuel to the stream of compressed air, comprising:

a. means for adding fuel to said stream of air in a manner providing a homogeneous air/fuel mixture at a predetermined temperature for generating said hot gases,

b. catalytic combustion means effective to sustain continuous, flameless oxidation of said mixture at a predetermined temperature thereby generating said hot gases,

c. heat exchange means for transferring heat from said hot gases to the stream of compressed air,

d. preheating means located upstream of said catalytic combustion means for raising the temperature of said mixture to said predetermined temperature in advance of sustained catalytic combustion, and

e. means for mixing a secondary air stream with both the primary air stream and duct mixture whereby a resultant homogeneous air/fuel mixture is attained which passes through said catalytic combu tion means. i

8. An apparatus as in claim 7, in which said air/fuel mixture has a ratio in excess of :1.

9. An apparatus as in claim 7, in which means are provided for defining a plenum zone between said heat exchange means and said catalytic combustion means and into which said primary and secondary air streams are introduced as well as said mixture from said duct, said mixing means being particularly characterized by swirl vanes located in the walls defining said plenum zone and effective to create a turbulent flow condition in said plenum zone. 

1. For use in a continuous combustion system having means for producing a primary stream of compressed relatively cool air to which heat and fuel is added to define a mixture for supporting combustion and thereby generating hot gases for driving a working element, apparatus for adding heat and fuel to the stream of compressed air comprising: a. means for adding fuel to said stream of air in a manner providing a homogeneous air/fuel mixture at a predetermined temperature for generating said hot gases, b. catalytic combustion means effective to sustain continuous, flameless oxidation of said mixture at a predetermined temperature, thereby generating said hot gases, c. heat exchange means for transferring heat from said hot gases to the stream of compressed air, said heat exchange means being comprised of a perforate rotating member having surfaces effective to contact separately both said hot gases and air stream during a single rotation for transferring heat therebetween, d. preheating means located upstream of said catalytic combustion means for raising the temperature of said mixture to said predetermined temperature in advance of sustained catalytic combustion by regulating said air/fuel mixture for supporting independent flaming combustion of said mixture upstream of said catalytic combustion means, said preheating means comprising a duct effective to separate a portion of said air stream and direct said portion through one radial region of said member, said preheating means being adapted to add fuel solely to said portion, said preheating means also having ignition means for igniting said regulated mixture and having control means for extinguishing said flaming combustion when said mixture attains said predetermined temperature for sustaining catalytic combustion, the ignition means being located proximate to at least one surface of said perforate member for igniting said mixture immediately downstream of said member.
 2. The apparatus as in claim 1, in which said perforate member is particularly defined as a disc having a ceramic matrix with flow passages therethrough disposed generally transverse to the axis of rotation of said disc, and said preheating means being arranged to anchor said flame to one side of said disc immediately adjacent the passages thereof which carry said mixture from said duct.
 3. The apparatus as in claim 2, which further comprises means for providing a cylinder of air to anchor said flame.
 4. The apparatus as in claim 1, in which the air/fuel mixture passing from said duct into said heat exchange member is between 8:1 and 30:1.
 5. An apparatus as in claim 1, in which said air stream is directed to pass through the regenerator on one side of the rotational axis thereof, said duct having an outlet adjacent said regenerator and offset counterclockwise relative to the rotation of said regenerator and from a line dividing said one generator side.
 6. An apparatus as in claim 1, in which the unseparated portions of said air stream is effective to pass through said regenerator and define a cylinder of air about the aiR/fuel mixture exiting from said duct, said air cylinder being effective to anchor the flame resulting from the combustion of said mixture.
 7. For use in a continuous combustion system having means for producing a primary stream of compressed relatively cool air to which heat and fuel is added for supporting combustion and thereby generating hot gases for driving a working element, apparatus for adding heat and fuel to the stream of compressed air, comprising: a. means for adding fuel to said stream of air in a manner providing a homogeneous air/fuel mixture at a predetermined temperature for generating said hot gases, b. catalytic combustion means effective to sustain continuous, flameless oxidation of said mixture at a predetermined temperature thereby generating said hot gases, c. heat exchange means for transferring heat from said hot gases to the stream of compressed air, d. preheating means located upstream of said catalytic combustion means for raising the temperature of said mixture to said predetermined temperature in advance of sustained catalytic combustion, and e. means for mixing a secondary air stream with both the primary air stream and duct mixture whereby a resultant homogeneous air/fuel mixture is attained which passes through said catalytic combustion means.
 8. An apparatus as in claim 7, in which said air/fuel mixture has a ratio in excess of 100:1.
 9. An apparatus as in claim 7, in which means are provided for defining a plenum zone between said heat exchange means and said catalytic combustion means and into which said primary and secondary air streams are introduced as well as said mixture from said duct, said mixing means being particularly characterized by swirl vanes located in the walls defining said plenum zone and effective to create a turbulent flow condition in said plenum zone. 